1. Field of the Invention
The present invention relates to a method of removing nickel(II), iron(III) and copper(II) from a chelant containing solution. More specifically, the invention relates to a method of precipitating the nickel from a chelant complex at a pH greater than 12.4 in the presence of the replacement calcium ion and dialkyldithiocarbamate precipitant.
2. Description of the Prior Art
Scale deposits on equipment in electric power generation, chemical processing, refineries, and/or other industrial plants where water is used for heating and/or cooling are frequently removed with commercial chemical cleaning solvents containing a chelant such as the alkylene polyamine polyacetic acid chelants (e.g., the polyvalent anions of ethylenediamine tetraacetic acid, EDTA, diethylenetriamine pentaacetic acid, DEPA, and hydroxylethylenediamine triacetic acid, HOEDTA) or certain organic polyfunctional acid chelants (e.g., the citrate anion). The reactions involved in the dissolution and chelation of metal scales are quite complex and involve a variety of chemical species. For example, spent chelant cleaning solution (i.e., after treatment and removal of scale from a ferrous metal surface), will frequently contain as much as several thousand parts per million (ppm) dissolved iron, up to several hundred ppm dissolved metals such as Cu, Cr, Ni or the like, and trace amounts of other metals as well as some free chelant. As such, the waste effluent from chelant cleaning solutions may be a hazardous or regulated waste.
The treatment or proper disposal of chelated metal ions found in waste effluent is now a recognized and accepted commercial necessity in view of current environmental regulations. Because of the presence of the chelated metal cation and associated stability of the complex, the commercially practical options are limited. Thus, it has been suggested that thermal decomposition of the chelant solution be employed or that such solutions be disposed of by deep well injection; however, neither alternative is considered cost effective. Furthermore, the known heavy metal recovery processes involving low temperature treatment of waste solutions with lime and caustic are ineffective, particularly when attempting to lower the nickel(II) concentration.